Control systems have been used within vehicles to control trailer brakes. Such systems generally activate in a manner correlated with actuation of the vehicle brakes to augment the braking force of the vehicle itself, which may be necessary or useful for towing larger or heavier trailers. Known systems provide a control signal for a coupled trailer brake system that is constantly proportional to the torque demand of the vehicle brakes (i.e. the position of the vehicle brake pedal), regardless of the actual braking effect achieved by the controlled actuation of the trailer brakes. While such systems may provide for adjustment of a gain applied to the brake torque demand signal to achieve the output trailer brake control signal, this gain adjustment is not responsive enough for a user to make adjustments during extended driving over varying speeds. Rather, such gain control is intended to be used upon a change in trailer load (either by removing payload or changing the trailer) before engaging in extended driving. As such, a test actuation input is also provided that can allow the user to test the result of the trailer brake control signal output at a current gain level to determine if the resulting brake application is too low (resulting in too low a brake torque being applied) or too high (which may result in locking of the trailer wheels due to excessive brake torque). Accordingly, a user can add or remove gain according to the test results. In some instances, a user may forget to adjust gain when needed, resulting in extended driving with either too little or too much trailer brake torque during braking. Further, as the overall brake torque (vehicle brakes and trailer brakes) needed to slow the combined vehicle and trailer varies with the speed of the combined vehicle and trailer (e.g., by requiring higher deceleration to slow over a comparable distance) and with road grade, a constant brake gain may result, for example, in excessive gain leading to wheel locking during braking from relatively lower speeds (e.g., less than 5 miles per hour). In a similar manner, similar trailer brake torque can result in lower deceleration from heavier trailers than with lighter trailers.